Muscles of the Anterior Abdominal Wall – 3D Anatomy Tutorial


Okay! So this is a tutorial on the muscles
of the abdominal wall. So I’m going to do this in two parts. The first part will be
on the muscles of the anterior abdominal wall and the second part will cover the muscles
of the posterior abdominal wall. So the muscles of the abdomen have several
different functions. They protect the viscera, which are the soften organs contained in the
abdominal cavity. They assist in breathing. In inspiration, the abdominal muscles relax
and allow the thoracic cavity to expand downwards. And in expiration, these muscles can contract
and push the viscera upwards. And also, contraction of these muscles is useful in coughing and
throwing up. Another function is to assist in defecation,
urination and giving birth. These are assisted by increasing the intra-abdominal pressure.
The pressure inside the abdominal cavities increase by a contraction of these muscles
and this can help force things out of the body like urine, feces and babies. So in the anterior abdominal wall, you’ve
got five muscles. These are all innervated by the anterior rami of spinal nerves T7-L1.
So you’ve got the rectus abdominis, the pyramidalis, the external oblique, internal oblique and
transversus abdominis. So I’ll just take you through these muscles now. So first, we’ve got this muscle here lying
right in the midline. It’s a paired muscle, which are separated by this white line running
down the middle. This white line is called the linea alba and this literally means ‘white
line’ in Latin. This linea alba is formed by the aponeurosis of the various abdominal
muscles. So the lateral three muscles (which I’ll come on to talk about), the external
and internal obliques and the transversus abdominis form this flat tendinous sheath
called aponeurosis which join together in the midline forming this linea alba. This
separates the two sides of the rectus abdominis muscle. So I’ve just isolated the muscle and we’ll
take a look at the origin and the insertion. The rectus abdominis muscle originates on
the pubic symphysis and pubic tubercle. Superiorly, it inserts along the costal margin (so the
costal cartilages) and also on the xiphoid process. So you can see the xiphoid process
here. So what this muscle does is it can flex the
vertebral column and it’s also involved in the other functions I mentioned at the start
of this tutorial. So a couple of other things to just point
out. You can see another white line which lies on the edge of the rectus abdominis muscle
on either side. This is called the linea semilunaris. And then you can see these horizontal intersections
which separate up the muscle bellies of the rectus abdominis. These are called tendinous
intersections. This can be seen on people who have low body fat and a lot of muscle
mass in the rectus abdominis. You can see in this picture the sort of surface
anatomy. You can see the linea alba down the midline and you can visualize the tendinous
intersections between the muscle bellies of the rectus abdominis. So the second muscle which lies anteriorly
in the abdominal wall is called the pyramidalis muscle. This is a tiny little muscle and it
isn’t always present. It attaches to the pubis inferiorly and in the midline, it attaches
to the linea alba. This muscle is innervated by the anterior ramus of T12, spinal nerve
T12. So I’ve just switched over a diagram to show
you this. What we’re looking at is the left side. This left side has been dissected away
and you can see one-half of the rectus abdominis muscle (so the right side of the rectus abdominis
muscle. Just at the bottom — so this is the pubis here and in the midline, you’ve got
the linea alba — the pyramidalis is this tiny little muscle here, which attaches from
the pubis to the linea alba. These two muscles, the rectus abdominis and
the pyramidalis muscle are actually enclosed in something called the rectus sheath. This
is a tendinous sheath formed by the aponeurosis of the three lateral muscles, which I’ll come
on to talk about next. So the upper three-quarters of the rectus
abdominis — so this area that I’m showing you here. The upper three-quarters are fully
enclosed by this rectus sheath, whereas the bottom quarter is actually — so only the
anterior surface is covered by the rectus sheath. So what I’m going to do now is I’m going to
switch over to a cross-section to show you this rectus sheath. So we’re now looking at a cross-section. This
is anterior. So you’ve got the rectus muscle (both half of the rectus muscle) and in the
middle, you’ve got the linea alba. And then laterally, you’ve got the three muscles, lateral
muscles, which I haven’t talked to you about yet. But just briefly, you’ve got the outer
muscle called the external oblique. The muscle below that is called the internal oblique.
And the innermost muscle is called the transversus abdominis. So as you can see, these muscles form this
flat tendon, which is known as an aponeurosis. So the aponeurosis of these three muscles
enclose the rectus abdominis muscle in what is called the rectus sheath. These aponeurosis
meet at the midline to form the linea alba. So this is a cross-section taken from the
upper three-quarters. So you can see how both surfaces of the rectus abdominis are enclosed.
So the anterior surface is covered; and so is the posterior surface. You can see how
the internal oblique splits. So it passes anteriorly and posteriorly behind the rectus
abdominis. So this next picture I’m showing you is from
the lower quarter, so the bottom quarter of the rectus abdominis muscle. You can see that
this sheath is slightly different because it doesn’t go behind the rectus abdominis
muscle. All three aponeurosis pass in front along the anterior surface of the rectus abdominis
muscle. So next we’ve got the muscles which sit laterally.
You know what these are. There are three muscles — the external oblique, internal oblique
and transversus abdominis (from superficial to deep). So first is this muscle here, the external
oblique. This muscle originates on ribs 5-12, which you can see here. It inserts inferiorly
on the iliac crest, the pubic crest and pubic tubercle in this area here. It inserts on
the midline on the linea alba and also on the xiphoid process up here. So you can see
how this muscle forms this flat tendon, which is called an aponeurosis. And you saw how
this aponeurosis surrounded the rectus abdominis muscles to form part of the rectus sheath. So the external oblique on either side joins
together in the midline, fuses to form the linea alba, which I showed you earlier. So if both muscles contract, it can flex the
trunk. But if one side contracts, it can laterally flex the trunk. It can flex the trunk to the
same side. The fibers of the external oblique pass inferomedially.
They pass downwards and towards the midline in this direction. So if we just remove the external oblique,
we’ve got the internal oblique which lies below it. These fibers pass in the other direction.
So they pass superiorly and medially. So one way of remembering the direction that
the fibers are oriented is to use the mnemonic ‘hands in pockets, hands on tits’. So how
does this help you to remember? So if your hands are in your pockets, imagine
your hands kind of like this. Pockets has the letters ‘e’ in it for external oblique.
The direction your forearm is the direction of fibers, so in this direction. For the external
oblique, you’ve got your hands in pockets, pockets has an ‘e’, external oblique. So the
fibers are in the inferomedial direction. Hands on tits — sorry for being crude, but
that’s the mnemonic — ‘hands on tits’, they’re up in that direction, so the fibers pass superiorly
and medially. So ‘tits’ has an ‘i’ in it, so ‘internal’ oblique. So the fibers are superomedial. So just going back to the external oblique
muscle, there’s one important thing I forgot to tell you about. The bottom part, the inferior
margin of the aponeurosis of the external oblique muscle actually forms the inguinal
ligament. I’ve just isolated it here and you can see
the bottom part of the aponeurosis forming this ligament, which runs from the anterosuperior
iliac spine here down to the pubic tubercle. That’s important to remember. Just going back to the internal oblique (and
you now know which direction the fibers pass), the internal oblique muscle originates on
the iliac crest and also on the lateral part of the inguinal ligament. And it inserts on
the lower ribs. So you can see these ribs, 10-12 (or bottom four ribs, 9-12) and it also
inserts in the midline in the linea alba. So it joins the other aponeurosis as you know.
And it also inserts in the pubic crest here. So the aponeurosis isn’t shown here, but you
know that it extends into the midline and joins the linea alba. So similar to the external oblique, when both
of the internal oblique muscles contract, it flexes the trunk. So it brings the truck
like this. And if one side, one muscle contracts, it laterally flexes the trunk. It brings the
trunk to the same side. So the last muscle we’ve got is the transversus
abdominis muscle. If I just remove the internal oblique, we’ve got this muscle that sits underneath
it. It’s called the transversus abdominis muscle because the fibers are oriented transversely,
so horizontally like this. Let’s just take a look at the origin. It originates
on the iliac crest (so you can see that here) and also on the lateral parts of the inguinal
ligament. And also, you can see the origin at the top on the costal cartilages. And then
again, it inserts on the linea alba and inferiorly, it inserts on the pubic crest. You can see its attachments here. So iliac
crest. The inguinal ligament is gone, but it attaches on the lateral part of the inguinal
ligament over here. It attaches on the costal margin and it attaches in the midline at the
linea alba and also inferiorly at the pubic crest. So those are the muscles of the anterior abdominal
wall. It’s pretty easy. You’ve just got five to remember. You’ve got two which sit anteriorly.
You’ve got the large rectus abdominis and you’ve got the tiny little pyramidalis. And
then you’ve got the three lateral muscles (the external and internal oblique muscles
and the transversus abdominis muscle).